Secondary lithium ion battery containing a prelithiated anode

ABSTRACT

The present invention provides a lithium ion battery that exhibits a significantly improved specific capacity and much longer charge-discharge cycle life. In one preferred embodiment, the battery comprises an anode active material that has been prelithiated and pre-pulverized. This anode may be prepared with a method that comprises (a) providing an anode active material (preferably in the form of fine powder or thin film); (b) intercalating or absorbing a desired amount of lithium into the anode active material to produce a prelithiated anode active material; (c) comminuting the prelithiated anode active material into fine particles with an average size less than 10 μm (preferably &lt;1 μm and most preferably &lt;200 nm); and (d) combining multiple fine particles of the prelithiated anode active material with a conductive additive and/or a binder material to form the anode. Preferably, the prelithiated particles are protected by a lithium ion-conducting matrix or coating material. Further preferably, the matrix material is reinforced with nano graphene platelets.

FIELD OF THE INVENTION

The present invention provides a secondary battery, particularly a rechargeable lithium-ion battery that comprises an anode containing a prelithiated anode active material.

BACKGROUND

The following list of references is closely related to the prior art. These references will be cited from time to time in the specification. They will be indicated as [Ref. xx] or simply [xx], where xx denotes a numeral.

LIST OF REFERENCES

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Takahashi, et al., “Secondary Battery,” U.S. Pat. No.     4,980,250, Dec. 25, 1990. -   21. C. K. Huang, et al., “Method for Fabricating Carbon/Lithium Ion     Electrode for Rechargeable Lithium Cell,” U.S. Pat. No. 5,436,093,     Jul. 25, 1995. -   22. J. K. Jacobs, et al., “Rechargeable Lithium Battery Having     Improved Reversible Capacity,” U.S. Pat. No. 5,721,067 (Feb. 24,     1998). -   23. E. Meissner, “Secondary Lithium-ion Cell with an Auxiliary     Electrode,” U.S. Pat. No. 6,335,115 (Jan. 1, 2002). -   24. A. P. Johnson, et al., “Rechargeable Lithium Ion Cell,” U.S.     Pat. No. 5,601,951, (Feb. 11, 1997). -   25. R. Herr, “Lithium Ion Cell,” U.S. Pat. No. 6,025,093 (Feb. 15,     2000). -   26. Y. Gao, et al., “Lithium Metal Dispersion in Secondary Battery     Anode,” U.S. Pat. No. 6,706,447, Mar. 16, 2004 and U.S. Pat. No.     7,276,314 (Oct. 2, 2007). -   27. Y. Gao, et al. “Lithium metal dispersion in electrodes,” US     Patent Application Pub. No. 2005/0130043 (Jun. 16, 2005). -   28. D. J. Burton, et al., “Method of Depositing Silicon on Carbon     Materials and Forming an Anode for Use in Lithium Ion Batteries,” US     Pat. Pub. No. 2008/0261116 (Oct. 23, 2008). -   29. J. Yang, et al., “Si/C Composites for High-Capacity Lithium     Storage Materials,” Electrochemical and Solid-State Letters,     6 (8) (2003) A154-A156. -   30. L. Chen, et al., “Spherical Nanostructured Si/C Composite     Prepared by Spray Drying Technique for Lithium Ion Batteries,”     Materials Science and Eng., B131 (2006) 186-190. -   31. H. Y. Lee and S. M. Lee, “Carbon-Coated Nano-Si Dispersed     Oxides/Graphite Composites as Anode Material for Lithium Ion     Batteries,” Electrochemistry Communications, 6 (2004) 465-469. -   32. K. Matsubara, et al., “Carbonaceous Material and Lithium     Secondary Batteries Comprising Same,” U.S. Pat. No. 6,733,922 (May     11, 2004). -   33. Z. Mao, et al. “Carbon-coated Silicon Particle Powder as the     Anode Material for Lithium Batteries and the Method of Making the     Same,” US 2005/0136330 (Jun. 23, 2005).

Concerns over the safety of earlier lithium secondary batteries led to the development of lithium ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials as the negative electrode (anode). The carbonaceous material may comprise primarily graphite that is intercalated with lithium and the resulting graphite intercalation compound may be expressed as Li_(x)C₆, where x is typically less than 1. In order to minimize the loss in energy density due to this replacement, x in Li_(x)C₆ must be maximized and the irreversible capacity loss Q_(ir) in the first charge of the battery must be minimized. The maximum amount of lithium that can be reversibly intercalated into the interstices between graphene planes of a perfect graphite crystal is generally believed to occur in a graphite intercalation compound represented by Li_(x)C₆ (x=1), corresponding to a theoretical specific capacity of 372 mAh/g [Ref. 1].

In addition to carbon- or graphite-based anode materials, other inorganic materials that have been evaluated for potential anode applications include metal oxides, metal nitrides, metal sulfides, and a range of metals, metal alloys, and intermetallic compounds that can accommodate lithium atoms/ions. In particular, lithium alloys having a composition formula of Li_(a)A (A is a metal such as Al, and “a” satisfies 0<a<5) has been investigated as potential anode materials. This class of anode active materials has a higher theoretical capacity, e.g., Li₄Si (3,829 mAh/g), Li_(4.4)Si (4,200 mAh/g), Li_(4.4)Ge (1,623 mAh/g), Li_(4.4)Sn (993 mAh/g), Li₃Cd (715 mAh/g), Li₃Sb (660 mAh/g), Li_(4.4)Pb (569 mAh/g), LiZn (410 mAh/g), and Li₃Bi (385 mAh/g). An anode active material is normally used in a powder form, which is mixed with conductive additives and bonded by a binder resin. The binder also serves to bond the mixture to a current collector. Alternatively, an anode active material may be coated as a thin film onto a current collector. On repeated charge and discharge operations, the alloy particles tend to undergo pulverization and the current collector-supported thin films are prone to fragmentation. Both are due to expansion and contraction of the anode active material during the insertion and extraction of lithium ions. This pulverization or fragmentation results in loss of the particle-to-particle contacts between the active material and the conductive additive or the contact between the anode material and its current collector. These adverse effects result in a significantly shortened charge-discharge cycle life [1].

To overcome the problems associated with such mechanical degradation, several approaches have been proposed [e.g., Refs. 2-18], including (a) using nano-scaled particles of an anode active material, (b) composites composed of small electro-active particles supported by less active or non-active matrices or coatings, and (c) metal alloying. Examples of more promising active particles are Si, Sn, and SnO₂. For instance, Umeno, et al. [Ref. 14-17] proposed an anode based on carbon-coated silicon prepared by thermal vapor decomposition. Although a specific capacity as high as 800-1,450 mAh/g was achieved, the capacity faded rapidly after 40 cycles. Hung [5] disclosed a method of forming a composite anode material. The steps include selecting a carbon material as a constituent part of the composite, chemically treating the selected carbon material to receive nanoparticles, incorporating nanoparticles into the chemically treated carbon material, and removing surface nanoparticles from an outside surface of the carbon material with incorporated nanoparticles. A material making up the nanoparticles alloys with lithium. The resulting carbon/nanoparticle composite anodes did not exhibit any significant increase in capacity, mostly lower than 400 mAh/g. In fact, most of prior art composite electrodes have deficiencies in some ways, e.g., in most cases, less than satisfactory reversible capacity, poor cycling stability, high irreversible capacity, ineffectiveness in reducing the internal stress or strain during the lithium ion insertion and extraction cycles, and some undesirable side effects.

The positive electrode (cathode) active material is typically selected from a broad array of lithium-containing or lithium-accommodating oxides, such as lithium manganese dioxide, lithium manganese composite oxide, lithium nickel oxide, lithium cobalt oxide, lithium nickel cobalt oxide, lithium vanadium oxide, and lithium iron phosphate. The cathode active material may also be selected from chalcogen compounds, such as titanium disulfate or molybdenum disulfate. These prior art materials do not offer a high lithium insertion capacity and this capacity also tends to decay rapidly upon repeated charging and discharging. A practical specific capacity of a cathode material has been, at the most, up to 200 mAh/g of the cathode active material, which rapidly decays as the charge-discharge cycling operation proceeds. In many cases, this capacity fading may be ascribed to particle pulverization or thin film fragmentation (analogous to the case of an anode material), resulting in a loss of electrical contact of the cathode active material particles with the cathode current collector. Since the cathode specific capacity is relatively low, there is a strong desire to make use of a cathode active material to its full capacity.

As a lithium-ion cell is charged and discharged, lithium is alternately stored in the cathode and in the anode, so that the total amount of cyclable charge corresponds to the amount of lithium shuttling back and forth between the two electrodes. When the cell is assembled, usually an amount of cathode active material is made to store the amount of lithium available for the subsequent cyclic operation. This selection of cathode, instead of anode, to pre-store the needed lithium was mainly due to the notion that cathode active materials, such as lithium cobalt oxide, are relatively stable in ambient air (e.g., against oxidation) compared to lithiated graphite. However, the notion that this amount of lithium that determines the battery capacity is totally supplied from the cathode, limits the choice of cathode active materials because the active materials must contain removable lithium. Also, delithiated products corresponding to LiCoO₂ and LiNiO₂ formed during charging (e.g. Li_(x)CoO₂ and Li_(x)NiO₂ where 0.4<x<1.0) and overcharging (i.e. Li_(x)CoO₂ and Li_(x)NiO₂ where x<0.4) are not stable [26]. In particular, these delithiated products tend to react with the electrolyte and generate heat, which raises safety concerns.

When the lithium-ion cell is assembled and filled with electrolyte, the anode and cathode active materials have a difference in potential of at most about 2 volts between each other. The difference in potential between the two electrodes, after the lithium-ion cell has been charged, is about 4 volts. When the lithium-ion cell is charged for the first time, lithium is extracted from the cathode and introduced into the anode. As a result, the anode potential is lowered significantly (toward the potential of metallic lithium), and the cathode potential is further increased (to become even more positive). These changes in potential may give rise to parasitic reactions on both electrodes, but more severely on the anode. For example, a decomposition product known as solid electrolyte interface (SEI) readily forms on the surfaces of carbon anodes, wherein the SEI layer comprises lithium and electrolyte components. These surface layers or covering layers are lithium-ion conductors which establish an ionic connection between the anode and the electrolyte and prevent the reactions from proceeding any further.

Formation of this SEI layer is therefore necessary for the stability of the half-cell system comprising the anode and the electrolyte. However, as the SEI layer is formed, a portion of the lithium introduced into the cells via the cathode is irreversibly bound and thus removed from cyclic operation, i.e. from the capacity available to the user. This means that, during the course of the first discharge, not as much lithium moves from the anode to the cathode as had previously been inserted into the anode during the first charging operation. This phenomenon is called irreversible capacity and is known to consume about 10% to 20% of the capacity of a lithium ion cell.

A further drawback is that the formation of the SEI layer on the anode after the first charging operation may be incomplete and will continue to progress during the subsequent charging and discharge cycles [26]. Even though this process becomes less pronounced with an increasing number of repeated charging and discharge cycles, it still causes continuous abstraction, from the system, of lithium which is no longer available for cyclic operation and thus for the capacity of the cell. Additionally, as indicated earlier, the formation of a solid-electrolyte interface layer consumes about 10% to 20% of the amount of lithium originally stored at the cathode, which is already low in capacity (typically <200 mAh/g). Clearly, it would be a significant advantage if the cells do not require the cathode to supply the required amount of lithium.

Takahashi, et al [20], Huang, et al [21], and Jacobs, et al [22] disclosed methods by means of which lithium is introduced into the anode active material in order to minimize the lithium consumption and thus the irreversible capacity loss. However, prelithiated carbon- or graphite-based anode active materials lead to electrodes which can be handled only under non-oxidizing and dry conditions, making practical production of lithium ion batteries difficult. Burton, et al. [28] prepared a lithiated carbon-silicon alloy by “evaporating lithium on the silicon-coated carbon material.” In this case, lithium is deposited on the surface of both silicon coating and carbon fibers, as opposed to inserting lithium into the bulk of either silicon or carbon. Again, lithium deposited on the exterior surface of an anode active material is just like a discrete piece of lithium metal, which is not air-stable and must be handled in an oxygen-free and moisture-free environment.

Meissner [23] disclosed a secondary lithium-ion cell which includes a lithium-intercalating, carbon-containing anode, a non-aqueous lithium ion-conducting electrolyte, and a lithium-intercalating cathode including a lithium-containing chalcogen compound of a transition metal, and a lithium-containing auxiliary electrode disposed in the cell to compensate for the irreversible capacity loss in the secondary lithium-ion cell. This auxiliary electrode is spatially separated from the electrolyte when the cell is positioned in a first orientation and contacts the electrolyte when the cell is oriented in a second position, for supplying additional lithium to the cell. Such an additional electrode makes the battery very complicated and difficult to make. Switching between two orientations is not a good strategy since it would complicate the handling of the battery and an average consumer would not pay attention to such a detail to ensure proper operation of such a battery.

The approach of using a sacrificial electrode, in addition to an anode and a cathode in a cell, was also proposed earlier by Johnson, et al. [24] and by Herr [25]. Again, this additional electrode further complicates the manufacture and operation of a resulting battery. The assembling operation of a battery containing a highly reactive lithium metal or alloy electrode must be handled in an oxygen-free and moisture-free environment.

Gao, et al. [26] disclosed a secondary battery containing an anode that is formed of a host material capable of absorbing and desorbing lithium in an electrochemical system and lithium metal dispersed in the host material. The lithium metal is a finely divided lithium powder and preferably has a mean particle size of less than about 20 microns. The host material comprises one or more materials selected from the group consisting of carbonaceous materials (e.g., graphite), Si, Sn, tin oxides, composite tin alloys, transition metal oxides, lithium metal nitrides and lithium metal oxides. The method of preparing such an anode includes the steps of providing a host material, dispersing lithium metal in the host material, and then forming the host material and the lithium metal dispersed therein into an anode. The lithium metal and the host material is mixed together with a non-aqueous liquid to produce a slurry and then applied to a current collector and dried to form the anode. Alternatively, the anode can be formed by chemical means by immersing the host material in a suspension of lithium metal in a non-aqueous liquid, and then formed into an anode. The approach of Gao, et al [26] has the following drawbacks:

-   -   (1) The anode is composed of an anode active material (e.g.,         graphite or Sn particles) and a discrete lithium metal phase         (fine Li metal powder particles) forming a mixture of two types         of particles. This implies that the anode still contains highly         active lithium particles that are sensitive to oxygen and         moisture and must be handled under very stringent conditions.     -   (2) The amount of lithium metal present in the anode is         prescribed to be no more than the maximum amount sufficient to         intercalate in, alloy with, or be absorbed by the host material         in the anode. For example, if the host material is carbon, the         amount of lithium is no more than the amount needed to make         LiC₆. This implies that the amount of lithium needed for the SEI         formation was not considered and, hence, the resulting battery         will suffer a capacity loss of 10%-20% after the first cycle.     -   (3) It is of significance to note that although Gao, et al [26]         mentioned in passing that the anode active material can be a         mixture of carbon, Sn, Si, etc, the mixture still further         contains lithium metal powder particles as an additional,         discrete phase that is un-protected. The resulting         multi-component, multi-phase mixture is still sensitive to the         presence of oxygen and water contents in the air, thus         significantly complicating the battery manufacturing operations.     -   (4) In a follow-on patent application, Gao, et al. [27]         suggested methods of lithiating an electrode prior to combining         electrodes and other components to form a battery. In all cases,         the electrode is composed of a mixture of discrete lithium metal         particles or wire screen and powder particles of a host         material, the latter being partially litiated. As shown in FIG.         1 of Gao, et al [27], the anode comprises discrete lithium metal         particles and a host material. Both the discrete lithium metal         particles and lithiated carbonaceous material (graphite) are         unstable in an oxygen- or moisture-containing environment.

Furthermore, Gao, et al. [26,27] have not fairly suggested how other anode active materials than graphite can be prelithiated in a controlled manner (e.g., without inducing a lithium coating on the surface of active material particles). In fact, no example was given to illustrate if or how other important anode active materials can be successfully prelithiated prior to battery production. No battery testing or electrochemical performance evaluation data was given in any of Gao's patents [26] or patent application [27] to demonstrate the advantages of their electrodes.

Therefore, there exists an urgent need for a secondary lithium ion battery that has one or more of the following features or advantages:

-   a) The battery does not contain a sacrificial electrode or an extra     electrode in addition to an anode and cathode in a cell; -   b) The battery comprises an anode that does not contain a discrete     lithium metal phase or lithium metal powder particles dispersed in     the anode; -   c) The battery contains an anode that comprises at least two types     of anode active material wherein at least one type of active     material is prelithiated (e.g., Si and Sn) and at least one type of     active material is not prelithiated (e.g., carbonaceous material,     such as graphite, hard carbon, soft carbon, surface-modified     graphite, chemically modified graphite, or meso-carbon micro-beads,     MCMBs). Prelithiated carbonaceous anode materials are unstable in     regular room air; -   d) The battery contains an anode that comprises at least a     non-carbon active material possessing an ultra-high lithium     absorbing capacity (e.g., Si that exhibits a specific capacity up to     4,200 mAh/g); -   e) The battery comprises an anode that contains an excess amount of     lithium (disposed inside a non-carbon anode active material, not on     its surface) to compensate for the formation of SEI layers, in     addition to providing enough lithium to intercalate into (or form a     compound with) a cathode active material. -   f) The battery features a long and stable cycle life due to an anode     that comprises fine, prelithiated active particles capable of     maintaining their integrity and their contact with the conductive     additive and the current collector. No further particle     pulverization takes place since they have been pre-pulverized.

Therefore, a need exists for an anode active material that has a high specific capacity, a minimal irreversible capacity (or a low decay rate), and a long cycle life. In order to accomplish these goals, we have worked diligently and intensively on the development of new electrode materials. These research and development efforts lead to the present patent application.

SUMMARY OF THE INVENTION

The present invention provides a lithium ion battery that exhibits a significantly improved specific capacity and much longer charge-discharge cycle life. In one preferred embodiment of the present invention, the battery comprises an anode active material that has been prelithiated and pre-pulverized. This anode may be prepared with a method that comprises (a) providing an anode active material (preferably in the form of fine powder or thin film); (b) intercalating or absorbing a desired amount of lithium into the anode active material to produce a prelithiated anode active material; (c) comminuting the prelithiated anode active material into fine particles with an average size less than 10 μm (preferably <1 μm and most preferably <200 nm); and (d) combining multiple fine particles of the prelithiated anode active material with a conductive additive and/or a binder material to form the anode.

The anode active material may be selected from the following groups of materials:

-   (a) Silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony     (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), and cadmium (Cd);     preferably of nanocrystalline or amorphous structure in a thin film     (coating) form. The coating is preferably thinner than 20 μm, more     preferably thinner than 1 μm, and most preferably thinner than 100     nm; -   (b) The alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi,     Zn, Al, or Cd, stoichiometric or non-stoichiometric with other     elements; and -   (c) The oxides, carbides, nitrides, sulfides, phosphides, selenides,     tellurides, antimonides, or their mixtures (e.g., co-oxides or     composite oxides) of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, or Cd.

Lithiation of an anode active material (e.g., Si particles or thin film) in a prior art lithium ion battery during the charging operation is known to induce a volume change to the anode active material up to 300-400% of its original dimension. Such highly dilated particles or thin film are extremely brittle and prone to pulverization or fragmentation. During repeated charge/discharge cycles, the anode active material tends to quickly become pulverized and the resulting fragments lose contacts with the conductive additive and the current collector. Those fragments are no longer active in providing electrons to the external circuit.

The presently invented method makes use of the notion that highly lithiated anode active material is very brittle and can be readily pulverized. The present invention has turned this drawback into an advantage. The presently invented approach entails prelithiating the anode active material to the extent that the prelithiated material becomes readily pulverizable into nano-scaled particles. The approach includes pre-pulverizing the prelithiated material into ultra-fine particles prior to incorporating these particles into an anode. The results have been most surprising in several aspects:

-   (a) We have surprisingly found that prelithiated materials can be     much more readily converted into nano-scaled particles as compared     to their non-lithiated counterparts. It is well known in the art     that the preparation of nano materials has been usually slow and     expensive. Prelithiation makes such a process much faster and less     expensive. Nano particles also mean a shorter distance for lithium     ions to penetrate in and out of the particles, resulting in a better     rate capacity for the battery. In other words, the electrode can     quickly store or release lithium and, hence, the battery can be     discharged or re-charged rapidly. This is a highly beneficial     feature for a battery that is intended for electric vehicle     applications, where a fast discharge rate capability (e.g., during     acceleration) is required. In all battery applications, a fast     re-charge rate is clearly a highly desirable feature. A driver can     quickly recharge a car battery in a matter of 10 to 60 minutes     instead of several hours. -   (b) Pre-pulverized, prelithiated particles, particularly those at     the nano scales, are no longer brittle and, for the most part, no     longer pulverizable in an anode when the battery undergoes repeated     charges and recharges. The issue of having undesired particle     pulverization goes away. -   (c) Even though prelithiated particles could shrink in volume during     the discharge process when lithium is extracted out of the anode     active material, we have further surprisingly found that shrinking     of prelithiated and pre-pulverized particles did not lead to loss of     contacts with conductive additives (e.g., carbon black or fine     graphite particles) and the current collector during the charge and     discharge cycles. Hence, the anode maintains a high capacity for a     much longer cycle life. In contrast, in the conventional Li-ion     battery electrodes (e.g., schematically shown in FIG. 2(A)),     although conductive additives could form a condition of percolation     where the particles contact one another to form a network of     electron-conducting paths, they are easily broken when the     electro-active particles expand or shrink during the Li ion     insertion or extraction steps. -   (d) Chan, et al [Ref. 19] proposed an anode active material, where     multiple Si nanowires were catalytically grown from an anode current     collector surface in a substantially perpendicular direction (FIG.     2(B)). There are severe drawbacks associated with these prior art     end-connected nanowire-based electrodes. For instance, each Si     nanowire is only connected to a current collector through a very     narrow contact area (diameter=89 nm) and, hence, the nanowire would     tend to detach from the steel current collector after a few volume     expansion-contraction cycles. Furthermore, if fragmentation of a     nanowire occurs, only the segment remaining in direct contact with     the current collector (e.g., steel plate in Chan, et al.) could     remain in electronic connection with the current collector and all     other segments will become ineffective since the electrons generated     will not be utilized. Additionally, the configuration of having     parallel nanowires perpendicular to a current collector is not     compatible with the current battery production practice, which     typically involves high-speed winding and packaging of multiple     layers of electrodes and separator. In contrast, the instant     invention does not require a change in the battery production     equipment or procedure. No additional major capital investment is     required.

Hence, the present invention provides a high-capacity, high cycle-life rechargeable lithium ion battery. This lithium-ion battery comprises a cathode, an anode, a separator disposed between the cathode and the anode, and electrolyte in ionic contact with both the cathode and anode, wherein the anode comprises fine particles of a first anode active material that contains lithium atoms prelithiated into the volume of at least one of the fine particles. The fine particles have a size less than 1 μm, preferably less than 500 nm, and most preferably less than 200 nm.

The anode may further comprise a desired amount of a second anode active material selected from particles of graphite, hard carbon, soft carbon, meso-carbon micro-bead, surface-modified graphite, carbon-coated graphite, or a combination thereof. The anode can comprise a conductive additive selected from the group consisting of carbon black, electro-spun nano fibers, carbonized electro-spun nano fibers, vapor-grown carbon or graphite nano fibers, carbon or graphite whiskers, carbon nano-tubes, nano-scaled graphene platelets, metal nano wires, metal-coated nano wires, carbon-coated nano wires, metal-coated nano fibers, carbon-coated nano fibers, and combinations thereof.

One preferred embodiment of the present invention is a lithium ion battery having an anode that is characterized by having one of the following features:

-   (1) The anode comprises fine particles of a first anode active     material that have been pre-lithiated and pre-pulverized; -   (2) The anode comprises fine particles of a first anode active     material that have been pre-lithiated and pre-pulverized. These fine     particles are protected by a thin coating material or embedded in a     matrix material. This coating or matrix material may be reinforced     with a nano-scaled filler such as a carbon nano-tube (CNT) or nano     graphene platelet (NGP); -   (3) The anode comprises fine particles of a first anode active     material that have been pre-lithiated and fine particles of a second     anode active material (e.g., graphite- or carbon-based material)     that are not pre-lithiated.

With any one of such anodes, it is not necessary for the cathode to pre-store the full amount of the required lithium. In fact, the cathode does not need to contain any lithium initially when the battery is assembled. This will further increase the battery capacity by up to 10-20%. These and other advantages and features of the present invention will become more transparent with the description of the following best mode practice and illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (A) Flow chart of a prior art process for preparing a lithium ion battery anode; and (B) Flow chart of an innovative method of producing an anode according to a preferred embodiment of the present invention.

FIG. 2 (A) Schematic of a prior art electrode where electro-active particles, conductive additive particles (e.g., carbon black or fine graphite powder), and a binder material are randomly mixed together; and (B) Schematic of a prior art anode composition composed of Si nanowires catalytically grown in a direction normal to a steel current collector according to Chan, et al. [Ref. 19].

FIG. 3 Specific discharge capacities of prelithiated, pre-pulverized Si particles with carbon coating and those of non-prelithiated Si particles.

FIG. 4 Ball milling results of prelithiated and non-prelithiated Si particles.

FIG. 5 The half-cell cycling behaviors of Comparative Sample-B, Sample-B-P, Comparative Sample-C, and Sample-C-P.

FIG. 6 The half-cell cycling behaviors of Sample-A, Sample-A-P, Comparative Sample-C, and Sample-C-P.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention is related to anode materials for high-capacity lithium batteries, which are preferably secondary batteries based on a non-aqueous electrolyte or a polymer gel electrolyte. The shape of a lithium metal or lithium ion battery can be cylindrical, square, button-like, etc. The present invention is not limited to any battery shape or configuration.

The present invention provides a lithium ion battery that is preferably prepared using a method that comprises (a) providing an anode active material (preferably in the form of fine powder or thin film); (b) intercalating or absorbing a desired amount of lithium into the anode active material to produce a prelithiated anode active material; (c) comminuting the prelithiated anode active material into fine particles with an average size less than 10 μm (preferably <1 μm and most preferably <200 nm); and (d) combining multiple fine particles of the prelithiated anode active material with a conductive additive and/or a binder material to form the anode. The starting anode active material in the form of a fine powder preferably has an average diameter less than 10 μm. If in a thin-film or plate form, the starting anode active material preferably has a thickness less than 10 μm.

As summarized in FIG. 1(A), in a conventional Li-ion battery, lithiation of an anode active material (e.g., Si particles) during the charging operation is known to induce a volume change in the anode active material by 300-400% of its original dimension. Such highly dilated particles are extremely brittle and prone to pulverization or fragmentation. During the repeated charge/discharge cycles, the anode active material tends to quickly become pulverized and the resulting fragments lose contacts with the conductive additive and the current collector. Those fragments are no longer active in providing electrons to the external circuit.

The presently invented approach (schematically shown in FIG. 1(B)) makes use of the notion that highly lithiated anode active material is very brittle and can be readily pulverized. The present approach turns this drawback into an advantage by (i) prelithiating the anode active material to the extent that the prelithiated material becomes readily pulverizable; and (ii) pre-pulverizing the prelithiated material into ultrafine particles prior to incorporating these particles into an anode. We have surprisingly found that pre-pulverized, prelithiated particles that are already smaller than 1 μm in size (particularly if less than 200 nm) are no longer brittle and, for the most part, no longer pulverizable in an anode when the battery undergoes repeated charges and recharges. The issue of having undesired particle pulverization in the anode went away.

Furthermore, even though prelithiated and pre-pulverized particles could still shrink in volume during the discharge process when lithium is extracted out of the anode active material, we have surprisingly found that the resulting anode was capable of maintaining a high capacity for a much longer cycle life compared with conventional anode structures. Presumably, shrinking of prelithiated and pre-pulverized particles did not lead to loss of contacts with conductive additives (e.g., carbon black or fine graphite particles) and the current collector during the charge and discharge cycles.

For a lithium ion battery, according to a preferred embodiment of the present invention, the starting anode active material (in a fine powder or thin-film form) may be selected from the following groups of materials:

-   (a) Silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony     (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), and cadmium (Cd);     preferably of nanocrystalline or amorphous structure in a thin film     (coating) form. The coating is preferably thinner than 20 μm, more     preferably thinner than 1 μm, and most preferably thinner than 100     nm; -   (b) The alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi,     Zn, Al, or Cd, stoichiometric or non-stoichiometric with other     elements; and -   (c) The oxides, carbides, nitrides, sulfides, phosphides, selenides,     tellurides, antimonides, or their mixtures (e.g., co-oxides or     composite oxides) of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, or Cd.

The anode active material for use in the presently invented lithium ion battery preferably includes at least one of silicon (Si), germanium (Ge), and tin (Sn) as an element. This is because silicon, germanium, and tin have a high capability of inserting and extracting lithium, and can reach a high energy density. The next preferred group of elements includes lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), and cadmium (Cd). When any of these two sets of elements are included as a primary element of an electro-active material (defined as being capable of absorbing and extracting lithium ions in the present context), which is prelithiated and pre-pulverized, the cycling stability of the resulting anode material can be significantly improved. Another preferred class of anode active material include the oxides, carbides, nitrides, sulfides, phosphides, selenides, tellurides, or their mixtures (e.g., co-oxides or composite oxides) of: (a) Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, or Cd for anode active materials; or (b) Co, Ni, Mn, V, Ti for cathode active materials. They can be readily produced in a thin-film or powder form.

Prelithiation can be accomplished in several different ways. A physical process entails depositing a Li coating on a surface of an anode active material substrate (e.g., thin Si plate or fine Si particles), followed by promoting thermally induced diffusion of Li into the substrate (e.g., into the interior of a Si plate or Si particles). A thin lithium layer can be deposited on the surface of a thin anode material sheet (e.g., Si plate) using a standard thin film process, such as thermal evaporation, electron beam evaporation, sputtering, and laser ablation. A vacuum is used during the deposition process to avoid reactivity between the atomic lithium and molecules of lithium reactive substances such as water, oxygen, and nitrogen. A vacuum of greater than 1 milli-Torr is desirable. When electron beam deposition is used a vacuum of 10⁻⁴ Torr is desired and a vacuum of 10⁻⁶ Torr is preferred to avoid interaction between the electron beam and any residual air molecules.

The evaporative deposition techniques involve the heating of a lithium metal to create a lithium vapor. The lithium metal can be heated by an electron beam or by resistive heating of the lithium metal. The lithium vapor deposits lithium onto a substrate such as a Si plate or Si particles. To promote the deposition of lithium metal the substrate can be cooled or maintained at a temperature lower than the temperature of the lithium vapor. A thickness monitor such as a quartz crystal type monitor can be placed near the substrate to monitor the thickness of the film being deposited. Alternatively, laser ablation and sputtering techniques can be used to promote thin lithium film growth on a substrate. For example, argon ions can be used in the sputtering process to bombard a solid lithium metal target. The bombarding knocks lithium off of the target and deposits it on the surface of a substrate. Laser ablation processes can be used to knock lithium off of a lithium target. The separated lithium is then deposited on a substrate.

A more preferred pre-lithiation process involves electro-chemically forcing Li atoms to migrate into the bulk of a Si plate or multiple Si particles under the influence of an electromotive force (emf). In a typical arrangement, again using Si as an example, a thin Si plate or thin Si rod is used as a positive electrode and Li metal sheet or rod as a negative electrode. This is similar to an electroplating procedure, but, surprisingly, Li atoms are capable of permeating into the bulk of the Si plate or rod (not just being plated on the surface). The surface-coated Li may be dissolved in a solvent and washed away. For electrochemical lithiation of Si particles, the particles may be confined in a porous container (e.g., fine metal mesh) that is permeable to electrolyte, but does not allow solid Si particles to escape. The fine metal mesh serves as a working electrode while a lithium metal rod or sheet serves as a counter electrode.

The aforementioned prelithiation processes are applicable to all the anode active materials discussed in the present specification, not just Si, although Si is used as an example to illustrate the best-mode practice. The anode active material preferably comprises silicon and the prelithiated particles comprise a prelithiated silicon Li₄Si, Li_(4.4)Si, or Li_(x)Si, wherein numerical x is between 1 and 4.4. These particles were found to be particularly suitable for size-reduction by using ball-milling to produce nano particles. The comminuted, prelithiated fine particles further preferably comprise a prelithiated silicon particle having a size less than 200 nm, more preferably less than 100 nm.

The step of providing an anode active material may comprise providing a doped semiconductor material. Such a doped semiconductor material may be selected from Si or Ge doped with n-type and/or p-type dopants. Commonly used n-type dopants are P, As, and Sb and commonly used p-type dopants are Al, Ga, and In.

The step of comminuting the prelithiated anode active material into fine particles may be accomplished using a wide range of grinding or milling process, such as air jet milling and ball milling, particularly high-intensity ball milling. Preferably, this step is carried out to the extent that the resulting fine particles have an average size less than 10 μm, preferably less than 1 μm, further preferably less than 200 nm, and most preferably <100 nm.

It may be noted that the electro-active material (e.g., prelithiated and pre-pulverized Si nano particles) may be coated with a carbonizable coating material (e.g., phenolic resin, poly(furfuryl alcohol), coal tar pitch, or petroleum pitch). The coating can then be carbonized to produce an amorphous carbon or polymeric carbon coating on the surface of these Si particles. Such a conductive surface coating can help maintain a network of electron-conducting paths during repeated charge/discharge cycles and prevent undesirable chemical reactions between Si and electrolyte from happening. Hence, the presently invented method may further comprise a step of coating a surface of the comminuted, prelithiated fine particles with a thin layer of carbon having a thickness less than 1 μm prior to step (d). The thin layer of carbon preferably has a thickness less than 100 nm. Such a thin layer of carbon may be obtained from pyrolization of a polymer, pitch, or organic precursor or obtained by chemical vapor deposition.

Alternatively, the electro-active material (e.g., prelithiated and pre-pulverized Si nano particles) may be dispersed in a protective matrix provided this matrix is lithium ion-conducting. A range of materials have been found to be conductive to lithium, including oxide (e.g., copper oxide), polymer (sulfonated conductive polymer), polymeric carbon, amorphous carbon, and meso-phase carbon. The prelithiated particles may be dispersed in a carbonizable precursor material (e.g., phenolic resin, poly(furfuryl alcohol), coal tar pitch, petroleum pitch, or meso-phase pitch). The resulting composite can then be subjected to a heat treatment convert the carbonizable precursor material into an amorphous carbon or polymeric carbon matrix (in which Si particles are dispersed, for instance). Such a conductive matrix coating can help maintain a network of electron-conducting paths during repeated charge/discharge cycles and prevent undesirable chemical reactions between Si and electrolyte from happening.

Further preferably, the protective matrix material (e.g., polymeric carbon, amorphous carbon, or meso-phase carbon) may be further reinforced with a high-strength nano-filler such as a carbon nano-tube (CNT) or a nano graphene platelet (NGP). An NGP is an individual nano graphene sheet (individual basal plane of atoms)or a stack of multiple nano graphene sheets. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate. In practice, NGPs are obtained from a precursor material, such as minute graphite particles, using a low-cost process, but not via flattening of CNTs. One of the cost-effective processes is exfoliation of graphite to produce graphite worms of loosely connected flakes, followed by separation of these flakes into isolated (unconnected) nano graphene platelets using mechanical means, such as air jet milling, rotating-blade shearing, and ultrasonication. These nano materials are cost-effective substitutes for CNTs or other types of nano-rods for various scientific and engineering applications.

After extensive and in-depth research and development efforts, we came to realize that NGPs (particularly those with a thickness <10 nm, preferably <1 nm) are very effective in enhancing the mechanical properties of a protective matrix (such as amorphous carbon or polymeric carbon, polymer, and metal oxide) in a lithium battery electrode. NGP-reinforced protective matrix materials are capable of cushioning the stresses-strains induced to an electro-active particle (e.g. prelithiated Si particles) during lithium insertion and extraction (discharge and charge) cycles. Surprisingly, NGPs were found to significantly enhance the structural integrity (strength and fracture resistance) of a range of protective matrix materials for electrodes, to the extent that a high specific capacity was maintained over a much larger number of cycles compared with those electrodes without an NGP-reinforced protective matrix. This could not be achieved with a carbon matrix containing conventional graphite particles. Preferably, the resulting nanocomposite composition (comprising prelithiated anode active particles, NGPs, and a matrix material) is made into a form of fine spherical or ellipsoidal particles.

Hence, in one preferred embodiment, the present invention provides a nano graphene-reinforced nanocomposite solid particle composition containing both NGPs and an electrode active material (prelithiated particles) dispersed in a protective matrix. The electrode active material is in the form of dispersed fine particles (particulates of various shapes, filaments, rods, tubes, and wires, etc.) with a dimension (e.g., diameter) smaller than 1 μm (preferably smaller than 500 nm, further preferably smaller than 200 nm). This nanocomposite solid particle composition is preferably in a form of fine particle (preferably <10 μm, more preferably <5 μm, and most preferably <2 μm) and is most preferably of a spherical or ellipsoidal shape. Such a shape is conducive to the formation of an electrode with a high tap density. A higher tap density means a better packing of electro-active material particles that results in a greater amount of active material per unit volume under an identical coating and laminating condition for electrode fabrication.

Hence, the present invention provides a lithium-ion battery comprising a cathode, an anode, a separator disposed between the cathode and the anode, and electrolyte in ionic contact with both the cathode and the anode, wherein the anode comprises: (A) fine particles of a first anode active material that has been prelithiated, having an average size less than 10 μm; (B) nano graphene platelets; and (C) a protective matrix reinforced by the nano graphene platelets wherein the fine particles of the first anode active material and the graphene platelets are dispersed in the protective matrix to form a nanocomposite containing x weight fraction of fine particles, y weight fraction of graphene platelets, and z weight fraction of protective matrix, where 2%<x<90%; 2%<y<90%; 5%<z<80%; and x+y+z=100%. After an extensive study we have discovered that the most preferred nanocomposite compositions are those with 10%<x<60%, 2%<y<30%, 5%<z<60%, and x+y+z=100%.

The nanocomposite solid particles may be made by (a) preparing NGPs from a laminar graphite material (such as natural graphite, artificial graphite, MCMB, graphite fiber, and carbon fiber); (b) preparing a precursor to a protective matrix material, (c) mixing the NGPs and an electro-active material (e.g., prelithiated Si nano particles, prelithiated nano-wires, prelithiated nano-rods, etc) with the precursor (possibly or optionally in a solvent or liquid medium) to form a suspension, (d) transforming the suspension into droplets (e.g., forming micron-sized solid particles using, for instance, an atomization or aerosol formation technique) and removing the solvent; and (e) converting the precursor into the desired protective matrix material (e.g., converting a polymer into a polymeric carbon via heat treatments). Alternatively, NGPs and electro-active material particles may be mixed with or coated by a monomer (e.g., a triazine-based compound), a polymer (e.g., sulfonated polyaniline), a ceramic material (e.g., a metal oxide) to form spherical solid particles that require no further chemical conversion. In some cases, additional protective material may be coated onto solid particles, e.g. further coated with an amorphous carbon matrix via chemical vapor deposition.

For the cases where prelithiated particles are coated with a protective coating or dispersed in a protective matrix (but in a form of fine composite particles), a conductive particle may or may not be necessary in the preparation of an anode structure. It would not be necessary if the protective matrix is carbon or NGP-reinforced carbon. If deemed desirable, acetylene black (AB), carbon black (CB), or ultra-fine graphite particles may be used as a conductive additive. Conductive additives may comprise an electrically conductive material selected from the group consisting of electro-spun nano fibers, carbonized electro-spun nano fibers, vapor-grown carbon or graphite nano fibers, carbon or graphite whiskers, carbon nano-tubes, nano-scaled graphene platelets, metal nano wires, metal-coated nano wires, carbon-coated nano wires, metal-coated nano fibers, carbon-coated nano fibers, and combinations thereof. A binder material may be chosen from polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), or styrene-butadiene rubber (SBR), for example. Conductive materials such as electronically conductive polymers, meso-phase pitch, coal tar pitch, and petroleum pitch may also be used as a binder. A typical mixing ratio of these ingredients is 80 to 85% by weight for the anode active material, 5 to 15% by weight for the conductive additive, and 5 to 10% by weight for the binder. The current collector may be selected from aluminum foil, stainless steel foil, and nickel foil. There is no particularly significant restriction on the type of current collector, provided the material is a good electrical conductor and relatively corrosion resistant. The separator may be selected from a polymeric nonwoven fabric, porous polyethylene film, porous polypropylene film, or porous PTFE film.

In a preferred embodiment of the present invention, step (d) may comprise combining multiple fine particles of prelithiated anode active material with a conductive additive and/or a binder material, plus a desired amount of another type of anode active materials selected from particles of graphite, hard carbon, soft carbon, meso-carbon micro-bead, surface-modified graphite, carbon-coated graphite, or a combination thereof.

Hence, a lithium ion battery may contain an anode that comprises at least two types of anode active material wherein at least one type of active material is prelithiated (e.g., Si and Sn) and at least one type of active material is not prelithiated (e.g., carbonaceous material, such as graphite, hard carbon, soft carbon, surface-modified graphite, chemically modified graphite, or meso-carbon micro-beads, MCMBs). Prelithiated carbonaceous anode materials are unstable in regular room air. The present invention enable the battery to contain an anode that comprises at least a non-carbon active material possessing an ultra-high lithium absorbing capacity (e.g., Si that exhibits a specific capacity up to 4,200 mAh/g). The battery comprises an anode that contains an excess amount of lithium (disposed inside a non-carbon anode active material, not on its surface) to compensate for the formation of SEI layers, in addition to providing enough lithium to intercalate into (or form a compound with) a cathode active material.

The present invention allows the excess amount of lithium to be stored in high-capacity anode active materials (there is no need to make use of the full capacity of Si, for instance). The capacity limitation is on the cathode side, rather than the anode side. The present approach obviates the need for the cathode to supply the needed lithium, thereby further reducing the needed initial weight of the cathode or increasing the cathode weight that can be incorporated in a cell. This strategy can increase the overall capacity of a lithium ion battery by another 10%-20%.

Hence, in summary, the present invention provides a high-capacity, high cycle-life rechargeable lithium ion battery. This lithium-ion battery comprises a cathode, an anode, a separator disposed between the cathode and the anode, and electrolyte in ionic contact with both the cathode and anode, wherein the anode comprises fine particles of a first anode active material that contains lithium atoms prelithiated into the volume of at least one of the fine particles. The fine particles have a size less than 1 μm, preferably less than 500 nm, and most preferably less than 200 nm.

The anode may further comprise a desired amount of a second anode active material selected from particles of graphite, hard carbon, soft carbon, meso-carbon micro-bead, surface-modified graphite, carbon-coated graphite, or a combination thereof. These carbon- or graphite-based anode active materials, by themselves, can be pre-lithiated, but the resulting products are not air-stable and must be handled in an oxygen-free and moisture-free environment. This would make mass-manufacturing of rechargeable batteries so much more challenging and expensive. By contrast, one may choose to pre-store some lithium in a non-carbon material (e.g., prelithiated Si) that enables convenient production of rechargeable lithium ion batteries that also have a much higher capacity.

The anode can comprise a conductive additive selected from the group consisting of carbon black, electro-spun nano fibers, carbonized electro-spun nano fibers, vapor-grown carbon or graphite nano fibers, carbon or graphite whiskers, carbon nano-tubes, nano-scaled graphene platelets, metal nano wires, metal-coated nano wires, carbon-coated nano wires, metal-coated nano fibers, carbon-coated nano fibers, and combinations thereof.

With such an anode, it is not necessary for the cathode to pre-store the full amount of the required lithium. In fact, the cathode does not need to contain any lithium initially when the battery is assembled. This will further increase the battery capacity up to 10-20%. These and other advantages and features of the present invention will become more transparent with the description of the following best mode practice and illustrative examples.

The positive electrode active material may be selected from a wide variety of oxides, such as lithium-containing nickel oxide, lithium-containing cobalt oxide, lithium-containing nickel-cobalt oxide, lithium-containing vanadium oxide, lithium iron phosphate, lithium manganese phosphate, lithium manganese-iron phosphate, and other lithium metal (or mixed metals) phosphate. Positive electrode active material may also be selected from chalcogen compounds, such as titanium disulfate or molybdenum disulfate. More preferred are lithium cobalt oxide (e.g., Li_(x)CoO₂ where 0.8≦x≦1), lithium nickel oxide (e.g., LiNiO₂), lithium manganese oxide (e.g., LiMn₂O₄ and LiMnO₂), lithium iron phosphate, lithium manganese-iron phosphate, lithium vanadium phosphate because these oxides provide a relatively high cell voltage and relatively good cycling stability.

Lithium cobalt oxide (LiCoO₂) is one of the most important cathode materials used in lithium-ion secondary batteries. LiCoO₂ and other similar lithium transition metal oxides, such as lithium manganese oxide, lithium nickel oxide, and lithium vanadium oxide, can be prepared by various methods using different lithium and transition metal sources. In general, bulk transition metal oxides are prepared by solid-state reactions, which involve repeated heat processes at high temperatures. Such processes generally afford the thermodynamically more stable phases and in general, microcrystalline materials are obtained. Lower temperatures and mild processing conditions may be used for several methods, such as co-precipitation, sol-gel process with/without template, synthesis by precursor, ion-exchange reaction and hydrothermal. These methods also result in particles with better control of morphology and smaller size. Other methods include flame spray pyrolysis, dehydro-freezing evaporation, supercritical dehydration, supersonic hydrothermal synthesis, and ultrasonic processing.

As an example, a process for producing lithium-cobalt oxide my include (a) mixing cobalt oxide having an average particle size of not more than 0.1 μm, with a lithium compound; and (b) calcining the obtained mixture at a temperature of 500 to 850° C. As compared to the conventional processes that begin with larger cobalt oxide particles (e.g., diameter >10 μm), such a process is advantageous in that lithium-cobalt oxide particles (1) can be produced with a short calcination time, (2) have a narrow particle size distribution, and (3) have a uniform small particle size.

The flame-spray pyrolysis method may include the steps of: (a) spraying minute droplets containing a solution of dissolved lithium salt and cobalt salt at room temperature; (b) atomizing the minute droplets through rapid expansion into a high temperature environment generated by combusting oxygen and hydrogen; (c) decomposing and oxidizing the atomized minute droplets thermally at high temperature to produce nano-sized oxides in gaseous phase; and (d) collecting the produced nano-sized composite oxides particles.

Lithium iron phosphate LiFePO₄ is a promising candidate of cathode material for lithium-ion batteries. The advantages of LiFePO₄ as a cathode active material includes a high theoretical capacity (170 mAh/g), environmental benignity, low resource cost, good cycling stability, high temperature capability, and prospect for a safer cell compared with LiCoO₂. A major drawback with this material is that it has very low electronic conductivity, on the order of 10⁻⁹ S/cm². This renders it difficult to prepare cathodes capable of operating at high rates. In addition, poor solid-phase transport means that the utilization of the active material is a strong function of the particle size. This major problem may be overcome by using a nano-scaled powder (to reduce the Li ion diffusion path and electron transport path distance) and doping the powder with a transition metal. Lithium iron phosphate (LiFePO₄) nano particles may be prepared by ball milling of conventional micron-sized particles, which may be prepared by a solid state reaction using LiOH.H₂O, (CH₃COO)₂Fe, and NH₄H₂PO₄ as raw materials. Additionally, Li_(1.3)Al_(0.3)Ti_(1.7)(PO₄)₃ materials, as an example of lithium mixed-metal phosphate, may be successfully prepared by the solution deposition using lithium acetate, aluminum nitrate, ammonium dihydrogen phosphate and titanium butoxide as starting materials. The resulting material may be ball-milled to sub-micron or nanometer scales. This is but one example of a host of complex metal phosphate-based cathode materials.

A wide range of electrolytes can be used for practicing the instant invention. Most preferred are non-aqueous and polymer gel electrolytes although other types can be used. The non-aqueous electrolyte to be employed herein may be produced by dissolving an electrolytic salt in a non-aqueous solvent. Any known non-aqueous solvent which has been employed as a solvent for a lithium secondary battery can be employed. A non-aqueous solvent mainly consisting of a mixed solvent comprising ethylene carbonate (EC) and at least one kind of non-aqueous solvent whose melting point is lower than that of aforementioned ethylene carbonate (hereinafter referred to as a second solvent) may be preferably employed. This non-aqueous solvent is advantageous in that it is (a) stable against a negative electrode containing a carbonaceous material well developed in graphite structure; (b) effective in suppressing the reductive or oxidative decomposition of electrolyte; and (c) high in conductivity. A non-aqueous electrolyte solely composed of ethylene carbonate (EC) is advantageous in that it is relatively stable against decomposition through a reduction by a graphitized carbonaceous material. However, the melting point of EC is relatively high, 39 to 4° C., and the viscosity thereof is relatively high, so that the conductivity thereof is low, thus making EC alone unsuited for use as a secondary battery electrolyte to be operated at room temperature or lower. The second solvent to be used in a mixture with EC functions to make the viscosity of the solvent mixture lower than that of EC alone, thereby promoting the ion conductivity of the mixed solvent. Furthermore, when the second solvent having a donor number of 18 or less (the donor number of ethylene carbonate is 16.4) is employed, the aforementioned ethylene carbonate can be easily and selectively solvated with lithium ion, so that the reduction reaction of the second solvent with the carbonaceous material well developed in graphitization is assumed to be suppressed. Further, when the donor number of the second solvent is controlled to not more than 18, the oxidative decomposition potential to the lithium electrode can be easily increased to 4 V or more, so that it is possible to manufacture a lithium secondary battery of high voltage.

Preferable second solvents are dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylene carbonate (PC), .gamma.-butyrolactone (.gamma.-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene and methyl acetate (MA). These second solvents may be employed singly or in a combination of two or more. More desirably, this second solvent should be selected from those having a donor number of 16.5 or less. The viscosity of this second solvent should preferably be 28 cps or less at 25° C.

The mixing ratio of the aforementioned ethylene carbonate in the mixed solvent should preferably be 10 to 80% by volume. If the mixing ratio of the ethylene carbonate falls outside this range, the conductivity of the solvent may be lowered or the solvent tends to be more easily decomposed, thereby deteriorating the charge/discharge efficiency. More preferable mixing ratio of the ethylene carbonate is 20 to 75% by volume. When the mixing ratio of ethylene carbonate in a non-aqueous solvent is increased to 20% by volume or more, the solvating effect of ethylene carbonate to lithium ions will be facilitated and the solvent decomposition-inhibiting effect thereof can be improved.

Examples of preferred mixed solvent are a composition comprising EC and MEC; comprising EC, PC and MEC; comprising EC, MEC and DEC; comprising EC, MEC and DMC; and comprising EC, MEC, PC and DEC; with the volume ratio of MEC being controlled within the range of 30 to 80%. By selecting the volume ratio of MEC from the range of 30 to 80%, more preferably 40 to 70%, the conductivity of the solvent can be improved. With the purpose of suppressing the decomposition reaction of the solvent, an electrolyte having carbon dioxide dissolved therein may be employed, thereby effectively improving both the capacity and cycle life of the battery.

The electrolytic salts to be incorporated into a non-aqueous electrolyte may be selected from a lithium salt such as lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃) and bis-trifluoromethyl sulfonylimide lithium [LiN(CF₃SO₂)₂]. Among them, LiPF₆, LiBF₄ and LiN(CF₃SO₂)₂ are preferred. The content of aforementioned electrolytic salts in the non-aqueous solvent is preferably 0.5 to 2.0 mol/l.

EXAMPLE 1

Summarized in FIG. 3 are the results of a study that illustrates the impressive advantages of the presently invented anode technology. The sample was prepared by using the prelithiated Si particles coated with amorphous carbon (13% after carbonization of phenolic resin) and bonded by PVDF as the working electrode and lightly lithium-plated Cu foil as the counter electrode and reference electrode. A specific discharge capacity as high as 2,500 mAh per gram of (Si+C) was maintained after 20 cycles. By contrast, non-prelithiated Si particles, with or without a carbon coating, exhibit a high initial capacity, but the capacity rapidly fades away as the number of repeated charge/discharge cycles increases.

EXAMPLE 2

FIG. 4 shows the results of ball-milling prelithiated and non-prelithiated Si particles. Clearly, prelithiated particles can be more easily reduced to sub-micron scales. This is a highly surprising, yet desirable result considering the fact that, in general, nano particles have been difficult and expensive to produce. The present invention unexpectedly discovers an efficient way to cost-effectively produce nano particles.

EXAMPLE 3

We have also investigated the cycling behaviors of several samples, including Si—NGP—C nanocomposite, Si—C nanocomposite (no NGP), and graphite-filled C—Si nanocomposite solid particle-based anodes (all containing Si nano particles). One of the samples prepared contains approximately 15% NGPs, 60% Si nano particles, and 25% carbon matrix and is herein designated as Sample-A. The corresponding sample containing Si nano particles that are prelithiated is designated Sample-A-P. For comparison purpose, a Comparative Sample-B was prepared by dispersing Si nano particles (approximately 70%) in a pitch matrix, as in Sample-A, but without the presence of any NGP. A corresponding sample containing Si nano particles that are prelithiated is denoted as Sample-B-P. A Comparative Sample-C was prepared by replacing NGPs with fine natural graphite particles (average diameter=3.5 μm) obtained by air-jet milling of natural flake graphite. A corresponding sample containing Si nano particles that are prelithiated is denoted as Sample-C-P.

FIG. 5 shows the half-cell cycling behaviors of Comparative Sample-B, Sample-B-P, Comparative Sample-C, and Sample-C-P. FIG. 6 shows the half-cell cycling behaviors of Sample-A, Sample-A-P, Comparative Sample-C, and Sample-C-P. The following observations can be made from these data:

-   (1) In general, the sample containing NGPs as a nano reinforcement     for the protective matrix or coating (e.g., carbon) is far superior     to the sample containing no NGPs as well as the sample containing     conventional graphite particles instead of NGPs (whether the Si nano     particles are prelithiated or not). This is a highly surprising     results in view of the results of the prior art studies on anode     materials based on Si and graphite particles dispersed in a carbon     matrix [Refs. 18,29-33]. The composite particles disclosed in the     prior art led to a specific capacity lower than 800 mAh/g (for up to     30-40 cycles only) [Ref. 18,29,30], lower than 600 mAh/g (up to 40     cycles) [Ref. 31], or lower than 460 mAh/g (up to 100 cycles) [Ref.     32]. These capacity values and cycling stability are not nearly as     good as what we have achieved (e.g., FIG. 6). It appears that carbon     by itself is relatively weak and brittle and the presence of     micron-sized graphite particles does not improve the mechanical     integrity of carbon since graphite particles are themselves     relatively weak. By contrast, the anode materials herein developed     have shown a reversible capacity of >2,000 mAh/g (per gram of total     composite weight counting Si, NGP, and C matrix weights) for over     350 cycles. Clearly, NGPs are capable of helping to maintain     structural integrity of a protective matrix, such as carbon, for an     anode active material. -   (2) The samples containing prelithiated active particles dispersed     in a protective carbon matrix exhibit a higher specific capacity for     a much longer cycle (better cycling stability) as compared with the     samples containing only active particles that have not been     prelithiated. Presumably, the prelithiated particles were already in     their higher or even highest volume expansion state when they were     dispersed in the carbon matrix during the anode fabrication stage.     When lithium was extracted from these anode active particles during     the discharge phase of battery cycling, these particles underwent     shrinkage. One would be concerned about the possibility that these     particles might de-wet or detach from the protective carbon matrix,     thereby losing contact with the carbon matrix (hence, providing no     electrons to the current collector). Quite surprisingly, this did     not appear to be the case. Presumably, these shrunk particles     remained in electronic contact with the carbon matrix. Furthermore,     during the re-charge phase, lithium was inserted back into the     interior of the anode active particles which got swollen back to an     original expanded volume state. This would not induce any     significant level of additional stress or strain to the surrounding     matrix and, hence, would not induce micro-cracking in the matrix for     a large number of charge-discharge cycles. This is a highly     surprising, but desirable consequence.

In summary, the present invention provides an innovative, versatile platform materials technology that enables the design and manufacture of superior anode materials for lithium ion batteries. This new technology has the following significant advantages:

-   (a) Prelithiated particles can be much more readily converted into     nano-scaled particles as compared to their non-lithiated     counterparts. In general, the production of nano particles is known     to be slow and expensive. Prelithiation brings the micron-scaled     particles to the state of high brittleness so that they can be     easily comminuted into nano-scaled particles rapidly and     cost-effectively. Nano particles also mean a shorter distance for     lithium ions to penetrate in and out of the particles, resulting in     a better rate capacity for the battery. -   (b) Pre-pulverized, prelithiated particles that are already     nano-scaled are no longer brittle and, for the most part, no longer     pulverizable in an anode when the battery undergoes repeated charges     and recharges. The issue of having undesired particle pulverization     has now been overcome. -   (c) Even though prelithiated particles could shrink in volume during     the discharge process when lithium is extracted out of the anode     active material, we have surprisingly found that shrinking of     prelithiated and pre-pulverized particles did not lead to loss of     contacts with conductive additives (e.g., carbon black or fine     graphite particles) and the current collector during the charge and     discharge cycles. Hence, the anode maintains a high capacity for a     much longer cycle life. In contrast, in the conventional Li-ion     battery electrodes, although conductive additives could form a     condition of percolation where the particles contact one another to     form a network of electron-conducting paths, they are easily broken     when the particles expand or shrink during the Li ion insertion or     extraction steps. -   (d) In the cases where the prelithiated anode active particles are     dispersed in a protective, yet conductive matrix (e.g., carbon), the     expansion-shrinkage operations did not appear to induce any     significant level of stress or strain in the matrix. The matrix had     already provided adequate room to accommodate the prelithiated     (pre-expanded) anode active particles. We feel that this is a very     clever strategy. -   (e) The present approach obviates the need for the cathode to supply     the needed lithium, thereby further increasing the cathode capacity     and avoiding the shortcomings associated with prelithiated cathodes. 

1. A lithium-ion battery comprising a cathode, an anode, a separator disposed between said cathode and said anode, and electrolyte in ionic contact with both the cathode and the anode, wherein said anode comprises fine particles of a first anode active material that has been prelithiated and pre-pulverized, having an average size less than 10 μm, and said first anode active material is not graphite-based or carbon-based.
 2. The battery of claim 1, wherein said prelithiated and pre-pulverized particles have an average particle size less than 1 μm.
 3. The battery of claim 1, wherein prelithiated and pre-pulverized particles have an average particle size less than 200 nm.
 4. The battery of claim 1 wherein said first anode active material is selected from the group consisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), and cadmium (Cd); (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements, wherein said alloys or compounds are stoichiometric or non-stoichiometric; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, or Cd, and their mixtures or composites; and (d) combinations thereof.
 5. The battery of claim 1 wherein said prelithiated anode active material is produced with a procedure comprising electrochemical lithiation of said active material.
 6. The battery of claim 1 wherein said anode active material comprises silicon and said prelithiated particles comprise a prelithiated silicon represented by a chemical formula of Li₄Si, Li_(4.4)Si, or Li_(x)Si, wherein numerical x is between 1 and 4.4.
 7. The battery of claim 1 wherein said prelithiated fine particles comprise a prelithiated silicon particle having a size less than 200 nm.
 8. The battery of claim 1 wherein said anode active material comprises a doped semiconductor material.
 9. The battery of claim 1 wherein said anode active material comprises a doped semiconductor material selected from Si or Ge doped with n-type and/or p-type dopants.
 10. The battery of claim 1 wherein said anode further comprises an amorphous carbon or polymeric carbon that forms a coating on a surface of said prelithiated fine particles or form a matrix in which a prelithiated particle is dispersed.
 11. The battery of claim 10 wherein said carbon coating has a thickness less than 1 μm.
 12. The battery of claim 10 wherein said carbon is obtained from pyrolization of a polymer, pitch, or organic precursor, or obtained by chemical vapor deposition.
 13. The battery of claim 1 wherein said anode further comprises a protective material that forms a coating on a surface of said prelithiated fine particles or form a matrix in which a prelithiated particle is dispersed.
 14. The battery of claim 1 wherein said anode further comprises a desired amount of a second anode active material selected from particles of graphite, hard carbon, soft carbon, meso-carbon micro-bead, surface-modified graphite, carbon-coated graphite, or a combination thereof.
 15. The battery of claim 1 wherein said anode comprises a conductive additive selected from the group consisting of carbon black, electro-spun nano fibers, carbonized electro-spun nano fibers, vapor-grown carbon or graphite nano fibers, carbon or graphite whiskers, carbon nano-tubes, nano-scaled graphene platelets, metal nano wires, metal-coated nano wires, carbon-coated nano wires, metal-coated nano fibers, carbon-coated nano fibers, and combinations thereof.
 16. The battery of claim 1 wherein said cathode does not contain a full capacity of lithium initially when the battery is assembled.
 17. A lithium-ion battery comprising a cathode, an anode, a separator disposed between said cathode and said anode, and electrolyte in ionic contact with both the cathode and the anode, wherein said anode comprises fine particles of a first anode active material that contains lithium atoms prelithiated into a volume of at least one of said fine particles and said fine particles have a size less than 1 μm.
 18. The battery of claim 17 wherein said fine particles have a size less than 500 nm.
 19. The battery of claim 17 wherein said fine particles have a size less than 200 nm.
 20. The battery of claim 17 wherein said anode further comprises a desired amount of a second anode active material selected from particles of graphite, hard carbon, soft carbon, meso-carbon micro-bead, surface-modified graphite, carbon-coated graphite, or a combination thereof.
 21. The battery of claim 17 wherein said anode comprises a conductive additive selected from the group consisting of carbon black, electro-spun nano fibers, carbonized electro-spun nano fibers, vapor-grown carbon or graphite nano fibers, carbon or graphite whiskers, carbon nano-tubes, nano-scaled graphene platelets, metal nano wires, metal-coated nano wires, carbon-coated nano wires, metal-coated nano fibers, carbon-coated nano fibers, and combinations thereof.
 22. The battery of claim 17 wherein said cathode does not contain a full capacity of lithium initially when the battery is assembled.
 23. The battery of claim 17 wherein said cathode contains no lithium initially when the battery is assembled.
 24. A lithium-ion battery comprising a cathode, an anode, a separator disposed between said cathode and said anode, and electrolyte in ionic contact with both the cathode and the anode, wherein said anode comprises: (A) fine particles of a first anode active material that has been prelithiated, having an average size less than 10 μm; (B) nano graphene platelets; and (C) a protective matrix reinforced by said nano graphene platelets wherein said fine particles of a first anode active material and said graphene platelets are dispersed in said protective matrix to form a nanocomposite containing x weight fraction of fine particles, y weight fraction of graphene platelets, and z weight fraction of protective matrix, where 2%<x<90%; 2%<y<90%; 5%<z<80%; and x+y+z=100%.
 25. The lithium ion battery of claim 24 wherein said protective matrix comprises amorphous carbon, polymeric carbon, or meso-phase carbon.
 26. The lithium ion battery of claim 24 wherein 10%<x<60%, 2%<y<30%, 5%<z<60%, and x+y+z=100%.
 27. The battery of claim 24 wherein said fine particles have a size less than 500 nm.
 28. The battery of claim 24 wherein said fine particles have a size less than 200 nm.
 29. The battery of claim 24 wherein said cathode does not contain a full capacity of lithium initially when the battery is assembled.
 30. The battery of claim 24 wherein said cathode contains no lithium initially when the battery is assembled. 